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III.—The Pre-Cambrian Rocks of Wales1
Published online by Cambridge University Press: 01 May 2009
Extract
In a recent article on the Pre-Cambrian Rocks of the British Isles in the Journal of Geology, vol. i., No. 1, Sir Archibald Geikie makes the following statement: “There cannot, I think, be now any doubt that small tracts of gneiss, quite comparable in lithological character to portions of the Lewisian rocks of the North-West of Scotland, rise to the surface in a few places in England and Wales. In the heart of Anglesey, for example, a tract of such rocks presents some striking external or scenic resemblance to the characteristic types of ground where the oldest gneiss forms the surface in Scotland and the West of Ireland.” To those who have followed the controversy which has been going on for nearly thirty years between the chiefs of the British Geological Survey and some geologists who have been working amongst the rocks in Wales, the importance of the above admission will be readily apparent; but as it is possible that some may be unable to realize what such an admission means in showing geological progress in unravelling the history of the older rocks in Wales during the past thirty years, a brief summary of the results obtained may possibly be considered useful.
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Footnotes
1
Paper communicated to the Geological Congress, Chicago, August, 1893.
References
page 397 note 1 The basal Cambrian conglomerates at St. Davids, which in places are over a hundred feet in thickness, often contain very large pebbles. This day (Aug. 17th) I measured two well-rounded blocks at the base of the conglomerate where it rests on the upturned edges of the Pebidian strata at Whitesand Bay, which gave the following dimensions:
Quartzite pebble with a slightly purplish tinge, 18 inches in length, 12 in width, and 9 in thickness. Quite near to the above was a well-rolled block of vein quartz, 17 inches in length, 16 in width, and 11 in thickness. On the same face dozens of pebbles 6 to 8 inches across were also visible; and amongst them were numerous somewhat smaller pebbles of slaty and compact volcanic ash, porcellanite, schistose felstone, etc., which had undoubtedly been derived from the underlying Pebidian rocks. The enormous quantity of quartzite, and the frequent presence of fragments of quartzose schists, in the conglomerate prove clearly that there must be a hidden ridge of such rocks (Arvonian) near at hand. The quartz-grains and bits of felspar which formed these quartzites and schists were primarily derived fram Granitoid rocks (Dimetian), such as are now seen at St. Davids, and on the coast at Poulthikey. In addition, therefore, to showing the marked unconformity on the Pebidian, these conglomerates yield indisputable proof of the exposure of three Pre-Cambrian groups in the St. Davids area when they were deposited. No amount of ingenious argument can alter these facts, and the evidence is perfectly clear to those who desire to reach it.